A first category of natural gas reservoir is known as a depletion gas reservoir. A second category of natural gas reservoir is known as a water drive reservoir.
In a depletion gas reservoir, the pore volume which contains natural gas remains constant over the duration of exploitation, so that the reservoir is “closed”. Recovery of natural gas from the reservoir is therefore accompanied by a decrease in the static reservoir pressure, and the extent of the ultimate recovery of natural gas from the reservoir depends upon the abandonment pressure. In a depletion gas reservoir, the ultimate recovery of natural gas may be as high as 70%-85% of the original gas in place (“OGIP”), and the extent of water production from the reservoir is typically very little or is absent altogether.
In a water drive reservoir, the pore volume which contains natural gas decreases over the duration of exploitation as natural gas is displaced by water. As a result, the reservoir pressure at abandonment of a water drive reservoir may remain relatively high. The water drive may be a lateral water drive or a bottom water drive. In the former case the water displaces the natural gas laterally or horizontally. In the latter case the water displaces the natural gas vertically upward. In both cases the water ultimately encroaches into production wells, usually in the lower parts of the pay interval. Also in both cases, the volumetric sweep efficiency is relatively low and the ultimate gas recovery is typically also relatively low (as low as 50%-65% of OGIP), due to the relatively low sweep efficiency and due to trapping of natural gas in the water invaded zone.
Enhanced recovery of natural gas from a depletion gas reservoir may be achieved by introducing a displacing agent into the reservoir. The displacing agent takes up a portion of the pore volume of the reservoir, which causes the natural gas to migrate within the reservoir and to occupy a smaller portion of the pore volume of the reservoir. The displacing agent can thus be used to drive the natural gas toward a production location and to pressurize the reservoir to a pressure above the abandonment pressure.
Carbon dioxide is derived both from natural sources and from man-made sources such as the burning of hydrocarbons and the carrying out of industrial processes. Carbon dioxide is the most abundant of the so-called “greenhouse gases”. It is generally believed that greenhouse gases may contribute to climate change and global warming. It has therefore become a significant environmental goal to limit the extent of carbon dioxide emissions into the atmosphere.
One strategy for limiting carbon dioxide emissions is to store or sequester carbon dioxide underground as an alternative to releasing it into the atmosphere. Carbon dioxide may be stored or sequestered in depleted oil and/or gas reservoirs, and may provide an added benefit of increasing the static reservoir pressure of the depleted reservoirs.
U.S. Pat. No. 7,172,030 (Horner et al) describes processes involving the injection into a subterranean reservoir of a waste gas stream containing nitrogen and carbon dioxide and in some cases oxygen as primary components. The injection of the waste gas stream into the reservoir may be performed for a variety of objectives. A first objective is the separation of carbon dioxide from the waste gas and retention of the separated carbon dioxide in a water presence in the reservoir. A second objective is increasing and/or maintaining the reservoir pressure to facilitate production of natural gas from the reservoir. A third objective is providing enhanced production of natural gas from the reservoir by displacement of natural gas towards one or more production wells. A fourth objective is causing some of the carbon dioxide to come into contact with and be dissolved in bitumen contained in the reservoir, thereby reducing the viscosity and improving the flow capability of the bitumen.
The processes described in U.S. Pat. No. 7,172,030 (Horner et al) are based upon the physical properties of the constituents of the waste gas stream relative to each other and relative to the physical properties of methane as a principal component of natural gas. As one example, the constituents of the waste gas stream are noted generally to have a relatively higher specific gravity and viscosity than does methane, which results in the constituents of the waste gas stream being advantageous agents for displacing or sweeping natural gas toward production wells. As a second example, the very high water solubility of carbon dioxide relative to the other constituents of the waste gas stream results in preferential dissolution of carbon dioxide in water which is present in the reservoir.